How to Insulate a Cedar Roof

Inspect your cedar roof from the inside to ensure that no leaks, damage or broken shingles or shakes exist. Repair any problem areas and seal any leaks to ensure a dry, warm environment.

Rcord the length of each roof rafter, using a tape measure. Use a circular saw to cut 2-by-8-inch lumber to fit these dimensions. Fasten one piece of 2-by-8 sleeper to the bottom end of each rafter to serve as an extension, which will allow for enough room to stuff the voids between the rafters with fiberglass insulation. Lay 2-by-8s wide side down against each rafter and a hammer and nails to fasten them in place. Set the sleeper so the overhang is even on both sides. This overhang will serve as a shelf on which your insulation rests.

Check that your soffit and ridge line vents are clear and operational before starting the insulation process. It is important that outside air circulates in the 1 1/2-to-2 inches of air space between the insulation and the underside of the cedar roof.

Cut R30 blanket insulation to fit the space between two rafters, using a razor knife. Slide the insulation into place between the rafters and allow it to settle against the sleepers installed in Step 2, with the foil backing facing downward. Use a staple gun to fasten the foil backing to the sleepers. Proceed to the next rafter space and repeat the process. Lay the foil from the second piece of insulation over that of the first and continue in the same manner until the entire roof has been insulated.

Cut rigid insulation board, using the razor knife, to fit the gaps between your rafters and sleepers if you prefer not to use rolled fiberglass insulation. Each sheet is top rated at R10, so you will require three sheets in each void to reach the recommended level of roofing insulation. Slide one sheet at an angle between the sleepers and allow it to settle into place. Repeat the process until three pieces have been installed in each void.

Install a sheet of polyethylene moisture blocker over your rigid, or rolled insulation, using a staple gun. Drive staples into the rafters to hold the sheeting in place.